The entire prostate gland is radiated when we treat the cancer.   The prostate normally produces some of the fluid in the ejaculation.   Radiation therapy has the side effect of damaging the glands in the prostate, so a lot less fluid is produced.  The ejaculation may be dry or nearly dry. In addition, you will probably be sterile (unable to father more children) after radiation, but this is not 100% guaranteed and should not be relied upon as a form of birth control. You can still usually have erections because the nerves and blood vessels that go to the penis are not as damaged as the prostate gland.

The prostate gland will end up having a lot of scar tissue.  It will shrink in size to about half its original weight within a couple years after finishing radiation.  The urethra (urine passage) passes through the canter of the prostate gland like the hole of a doughnut.  Sometimes this passage can widen, other times it can shrink after radiation.  In summary, the prostate gland is heavily damaged from radiation and does not work normally afterwards.